I will share our experiences for your consideration. A few notes. Our kids are 3 and 5. And, we have a travel trailer so we are stuck in the tow vehicle. We have found 400 miles a day to be a sweet spot for daytime driving. I do more when an over night is involved because the kids are sleeping. The iPad only gets a so far for treating boredom. We have done the truck stop/parking lot overnights. In most cases this option leaves little activities for the kids. We have started to opt for slightly shorter driving days and a late afternoon campground overnight. For us (me mostly), this has maintained my sanity by giving everyone a chance to get some energy out.

As for RV prep, we don't do anything different. We always keep fresh water on board for the bathroom. Early in my parenting career when I thought i could do 12 hour days like I was 23 again, I did carry a generator to give power for truck stop overnights in hot weather. With motorhome that isn't an issue for you.

We too top out at about 500 miles per day. Portable DVD players are pretty cheap. We load up on Redbox movies and stop when the kids get cranky. They can earn spending money for treats when we stop for gas.

We load up the RV when we travel long distances - empty black and grey. Full propane, fresh water and fridge.

If we need to push it, then one of us will make lunch/dinner while we're driving as long as it's straight steady freeway.

All the above... And, bikes may be a plus or a golf cart (they may be avail off site at a reasonable price). For travel, 8-9 hrs is abt my limit, I did 12 hrs a couple of years back and promised "never again". If you have time constraints then one night stops may be required, if not then search for intermediate stops for 2 nights or more to add to the experience.
The bride and I did WDW a couple of years ago and stayed in Ft Wilderness. An excellent CG! We bought a delux meal plan which worked well for us. Pick your plan and options and enjoy. We also spent a day th Universal with the main objective to visit the Harry P experiences.
Safe travels!

We traveled from Pennsylvania to WDW. Kids were 8, 11, and 13, We stopped in Wally Mart parking lots for the night. Truck stops were too noisy. We would call ahead and get the managers permission. We would also ask if they have or had issues with RV's in the parking lots. Most said no. But we have talked with a few in our travels that told us they had some issues with theft on the RV's. We went to a different Wally Mart. We would then go in the store and look around. Stock up on anything we missed. Kids liked doing this. We did this down and back. Because we left home at 10:00 PM to avoid the heavy traffic on the Washington Beltway, our 1st stop was in Southern Virginia for the night. Second night we stopped in Georgia. We went to the Holy Land Experience at 4655 Vineland Rd, Orlando, FL 32811. We called ahead and they allowed us to camp in their parking lot overnight.
Very pleasant trip. Fort Wilderness was nice although we were not there much as our days were at WDW. We went both ways. Boat trip to WDW and the Bus.
Recommend getting the express pass for some of the rides that have long lines.
Have a great time. We did.

My wife and I team drive when we're going cross country and time is at a premium. Not sure if that's an option.. but we do non-stop trips from Denver to the west coast, only stopping for fuel and to stretch our legs. We usually do 4 hour 'shifts' so nobody gets burned out. I like driving overnight when the traffic is a lot more reasonable. We keep our thermos full of hot coffee.

We are both a little haggard when we show up at our final destination and usually spend our first full day on site recovering from the journey.

If I was retired and wasn't burning vacation time getting from point a to b, I'd take my time.. but I'm always just 'wanting to get there' when my paid time off is at stake.

ETA- we keep our 14 year old daughter as occupied as possible, but normally we just 'encourage' her to sleep, read, watch a few movies, etc..